1. Field of the Invention
The present invention relates to a one-component dispersion.
2. Discussion of the Prior Art
Nowadays, high performance adhesive bondings are achieved according to the state of the art with reactive adhesives. For this purpose, a series of different adhesive systems is known. Due to their high reactivity, polyisocyanates are used for many applications in this field.
There are, for example, isocyanate-terminated polymers which cross-link after the application with moisture. The disadvantage of these systems is that curing takes a long time since cross-linking is dependent on the diffusion rate of water. Furthermore, cross-linking can remain incomplete in the case of very thick layers.
Two-component adhesives are mixed immediately before their application, which comprises an additional step for the user. Moreover, while preparing the two-component systems, errors may occur in proportioning or in mixing.
Further, one-component systems are known which may be formulated by blocking isocyanates with monofunctional reactants. Said blocking is thermally reversible, so that at elevated temperatures the blocking agent is separated and the released isocyanate forms a thermostable bond with the reactive polymer groups by cross-linking.
The disadvantage of these systems is that, if said blocking agent is volatile at application temperature, it vaporizes and may thus burden the environment or, if said blocking agent remains in the adhesives, it may lead to a deterioration or degradation as to the quality of said adhesives.
European referance 0 204 970 discloses a process for the production of stable dispersions of fine powdered polyisocyanates in a liquid by treating said polyisocyanates with stabilizing agents. In order to produce storage-stable dispersions of polyisocyanates and especially solid polyisocyanates and in order to prevent a premature, undesired reaction with the dispersion medium, the dispersed isocyanates are deactivated on their surface. They then possess a so-called retarded reactivity.
European referance 0 204 970 further discloses the use as a cross-linker for said stable dispersions of fine powdered polyisocyanates produced according to the process of said document. The stabilization of the polyisocyanates is obtained by the application of a polymeric shell. The compounds used to enable this polymeric shell are called stabilizing agents. The deactivated polyisocyanates, which comprise said polymer shell, are called stabilized polyisocyanates. The disadvantage here is that the burst or break-up of the polyurea capsules and thus cross-linking occurs at a temperature between 70 and 180xc2x0 C.
Thus, the object of the present invention is to provide a one-component dispersion cross-linking at low temperatures.
Said problem is solved by a one-component dispersion comprising deactivated isocyanates as a cross-linking component. Applicants surprisingly found out that the use of encapsulated or deactivated isocyanates, which are known per se, as cross-linking agents in aqueous dispersions leads to storage-stable systems cross-linking already at temperatures lower than 70xc2x0 C.
It is assumed that the self-adhesion forces arising during the formation of the polymeric film mechanically destroy the polymeric shells of the isocyanates, thus leading to a cross-linking reaction which can be shown on the basis of the increase of film cohesion or the reduction of film elasticity, respectively.
The one-component dispersion according to the present invention has the advantage that it cross-links at relatively low temperatures, especially at room temperature, thereby avoiding the disadvantages of stable prior art one-component systems. Compared with prior art two-component adhesives, the one-component dispersions of the present invention provide the advantage that they exclude the possibility of errors in application by the user, e.g. proportioning or mixing errors.
According to one embodiment of the present invention, the amounts of deactivated isocyanates in the one-component dispersions of the present invention are within the range of from 0.1% by weight up to 20% by weight, and especially of from 1% by weight up to 10% by weight.
The one-component dispersions of the present invention may be used in a multitude of applications, e.g. as a paint or a lacquer, as a coating, as a sealing material, as a casting material and/or as an adhesive.
When used as an adhesive, a suitable dispersion adhesive according to the present invention may contain, for example, Dispercoll(copyright) KA 8758. According to the present invention, suitable isocyanates are, for example, MDI or TDI, such as TDI-urethdione or TDI-urea. These isocyanates are deactivated according to the method known per se, and the resulting deactivated isocyanates are then added to an appropriate polymer such as a dispersion adhesive, while stirring. When the deactivated isocyanate is homogeneously and smoothly incorporated, the one-component dispersion of the present invention is ready to be used.
The one-component dispersion of the present invention can advantageously be used, for instance, for the bonding of three dimensional furniture fronts. These types of furniture fronts are produced in such a way, nowadays, that a plastic film or foil is pressed to a contoured medium density fiberboard carrier under pressure while applying heat, so that the film or foil takes the shape or the contours of the medium density fiberboard surface. The friction-type bond between the two substrates requires an adhesive which has previously been applied to the carrier. It is known that in order to realize high performance bonding with good heat stability, two-component polyurethane dispersion systems are used, wherein the aqueous dispersion is mixed with the water-emulsible polyisocyanate immediately before use. Due to proportioning errors or insufficient homogenization an optimal heat stability cannot be reached. Furthermore, the pot-life of these two-component systems is limited to a few hours only.
The above-mentioned disadvantages can advantageously be avoided by using the one-component dispersion adhesive of the present invention.